Page 128 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c006 Final Proof page 117 2.9.2005 9:38am

Microtechnologies for Space Systems 117

technology will be implemented to transport the waste heat towards the aft shroud’s
exterior, where it is radiated into outer space. The primary challenge with this type
of cooling scheme is the development of an efficient thermal interface between the
CPL and the external radiator.
This heat transfer challenge provides a unique opportunity for the first use
12
of carbon nanotubes (CNTs) in space. The CNTs will be incorporated within
a thermal interface kit for the HST instrument system as the only materials
choice that satisfies the following requirements. The interface should withstand the
harsh vacuum and radiation environment of space, have good electrical isolation, be
mechanically compliant, be abrasion tolerant for manual assembly by astronauts, and
not contaminate the spacecraft or its instruments. Competing solutions based on
polymer materials have the needed flexibility, however, these are either not good
thermal conductors or could contaminate the HST by outgassing high-vapor pressure
compounds. CNTs on the other hand are flexible (10% linear elasticity), strong
(Young’s modulus greater than 1 TPa) and with high thermal conductivity (theoret-
ical axial conductivity of 6000 W/m-K). The CNT-based thermal interface is a joint
development of NASA’s GSFC and the Ames Research Center (ARC) in collabor-
ation with the Applied Physics Laboratory (APL) at the Johns Hopkins University.
ARC and APL are aligning arrays of CNTs, and optimizing their characteristics for
the needs of the HST project. 13 GSFC will be responsible for the testing and
integration of the novel thermal interface. The CNT-based thermal interface will be
a 6-square-inch, copper-backed thermal conductor, with approximately 4 billion,
40-mm-tall nanotube ‘‘bristles.’’

6.2.6 RF MEMS SWITCH
Rockwell Science Center (RSC) developed a MEMS-based RF switch that was
14
flown successfully on two PICOSAT missions. The RSC devices were metal-
contacting switches that were fabricated using low-temperature surface-microma-
chining techniques. RF MEMS switches are an exciting alternative to the conven-
tional semiconductor field effect transistor-based switches, since they overcome
several of the shortcomings of semiconductor switches. Among other advantages,
RF switches have low mass, low power and small size, low RF insertion loss, high
isolation, and high intermodulation product. Additionally, micromechanical
switches are inherently radiation tolerant and robust for space applications. As
shown in Figure 6.5, 14 the RSC switch is a microrelay consisting of a metal shunt
bar that is suspended over a gap in the RF conductor line. Contact is made by
electrostatically attracting the shunt bar down to the RF conductor by means of
voltage applied to two drive capacitors attached on either side of the shunt bar. The
mechanical restoring force is provided by cantilevered silicon dioxide springs.
The RSC switch has a low insertion loss of ~ 0.2 dB for the range between dc
and 40 GHz. It also has very good isolation of greater than 60 dB at DC and ~ 25 dB
at 40 GHz. Actuation voltages are generally around 80 V with settling times
following on or off and off or on transitions being about 10 ms. The relay materials

© 2006 by Taylor & Francis Group, LLC